The overarching hypothesis which drives this work is that horizontal connections in the cortex mediate a host of basic visual phenomena and that they do so by modifying the classical receptive fields of cortical neurons in accordance with the visual context surrounding their receptive fields. these processes appear to support segmentation based on differences in disparity, texture, and motion and to support more subtle contextual tasks such as increasing the saliency of such geometry's as smooth curves and illusory contours. The proposed work will identify the regions of space from which contextual effects are manifested, and will test the hypothesis that it is these horizontal cortical connections which are responsible for the contextual effects. The proposed experiments will also discriminate between two hypothetical bases of the contextual effects, i.e., whether it is a change of gain in the classical receptive field mechanism or an actual change in the kernels describing the input-output behavior of the classical receptive field. In comparison with psychophysical effects, the association field of cortical cells will be explored in detail in an effort to account for the special significance of coaxial stimuli outside the classical receptive field and their possible role in the responses seen to illusory contours. In addition, manifestations of the responses elicited from within the classical receptive field in the time domain will be examined. Collectively, these experiments will vastly enhance the understanding of lateral interactions in cortex.